When we first began the GAPS diet we were still needing to eat low histamine, so we were not able to eat the fermented foods that it suggested. However, as we healed, we not only lost our sensitivity to ferments but they became an important part of our further healing. We began to focus on increasing the amount of fermented foods in our diet, both as a source of probiotics and as a source of vitamins that the healthy gut flora would normally make for us inside our bodies. We have found that they have helped us tremendously and that we really enjoy them. They are generally easy to make and much less expensive than buying probiotics, although we do still do that as well.
Here are some resources:
Fermentation: A wild way to make food come to life (an article from the Washington Post, with recipes at the end)
This post from the blog Girl in an Apron gives some information about making milk kefir, including a list of the microflora it contains, which is great to know. When I make kefir I personally skip the part where she warms the milk in a water bath.
Here are some basics on fermenting vegetables from Cultures for Health. This video with Donna Gates of The Body Ecology Diet goes into more detail about making fermented vegetables.
Sweet success with sauerkraut is great for trouble-shooting making sauerkraut.
This page from Organic and Thrifty gives some great ideas for introducing fermented foods to kids who may not be excited about eating them. Here are some more ideas for using kefir.
Dom's kefir site has pretty much all of the information you may want about kefir, including its history and how to make all sorts of cheeses with it.
The Nourished Kitchen blog has a nice post about the variety of cultured dairy options out there. Some of these I had not heard of before and am eager to try. The site Cultures for Health has resources to help with making a many of these options (including how to make non-dairy yogurt).
If you're avoiding dairy, you can make water kefir, which is essentially a natural soda (more on making water kefir here). That link has videos to help you get started. Another delicious fermented drink is kombucha, which is a fermented tea drink. Beet Kvaas is another option for a non-dairy fermented drink. The flavor is very strong though, and is more of an acquired taste. Jun tea is essentially a version of kombucha made with green tea and honey that is more mild. Unfortunately I had to give up both kombucha and beet kvaas because both are very high in oxalate.
This site has instructions for converting milk kefir grains to water kefir grains.
How to Make Honey Ginger Switchel
Lacto-fermented (pickled) vegetables are delicious too, often easy to make, and can be made in a tremendous variety depending on taste. This article from the Weston A. Price Foundation gives some background information and general ideas about using lacto-fermentation with vegetables and fruits. Here are instructions for making lacto-fermented turnips and beets. There are many, many recipes for fermented vegetables.
Kimchi
Homemade Kimchi
Recipe for pickled pea pods
Fermented Green Tomatoes and Hot Peppers
Cultured carrot sticks
Lacto-fermented turnips and beets
Fermented jalapenos
Fermented salsa
Lacto-fermented guacamole
Lacto-fermented hummus
Lacto-fermented ketchup
Lacto-fermented mayonnaise
Cultured fruit leather
Lacto-fermented orange marmalade
Moroccan Preserved Lemons
Fermented Garlic Honey
Fermented raspberry preserves
Lacto-fermented applesauce
5-spice apple chutney
Fermented peach chutney
How to make cultured butter
These two books are great resources:
Nourishing Traditions by Sally Falon
Wild Fermentation by Sandor Katz